TI-84 Plus CEs made within the last few years have a tenancy to get stuck in a boot loop when their battery dies. I didn't understand why this was so I did some investigating.

I thought the charging circuit was completely independent of the OS but this doesn't seem to be the case. When the OS is loaded the charge limit is 500ma. However, whenever the OS isn't loaded (i.e. when validating OS, holding RAM reset button, or on the OS reinstall screen) charging is throttled to less than 100ma.

I measured how much current is pulled (when the OS isn't loaded) on few different TI-84 Plus CEs:
  • L-0315: 97ma (This may not be accurate, read below)
  • L-1215A0: 88ma
  • L-0122W: 87ma

Left: L-0315 | Right: L-0122W (click for full size)
Not pictured: L-1215A0

For such a low powered device, I figured ~90ma would be enough to let the calculator do whatever it wanted and still charge the battery. However, I observed that just as the OS starts to load (after validating OS) the calculator stops charging completely for 1-2 seconds. This means it's likely the battery is getting charged, but the calculator booting just takes too much power and drains what little charge was just put in.

My theory is that if the screen is unplugged, the calculator will put more power into the battery than it takes to boot. To see if this theory has any hope of working I needed to test the power draw from the battery with and without the screen.

Testing L-0315
Date: March 2015
Revision: pre-A
OS: 5.3.0

This calculator is heavily modified. Based off the current draw results above, I don't trust any tests to be accurate.

Testing L-1215A0
Date: December 2015
Revision: A0
OS: 5.3.1

With the screen: Spikes up to 68ma then settled at: 52ma.
Without the screen: Spikes up to 39ma then settled at 24ma.

The screen is using around 68 - 39 = 29ma.
That's 29 / 68 = 42% of the battery's current draw when booting and over 29 / 52 = 55% when idling.

So of the 88ma of USB current, the battery is getting:
With the screen: 88 - 68 = 20ma or 20 / 88 = 22% of the USB current.
Without the screen: 88 - 39 = 49ma or 49 / 88 = 55% of the USB current.

Testing L-0122W
Date: January 2022
Revision: W
OS: 5.6.0

With screen: Spikes up to 53.8ma then settled at 51.0ma
Without screen: Spikes up to 20.8ma then settled at 17.7ma

I have to commend TI's engineers for lowering the overall power draw overall significantly! However, clearly it's still not enough to charge a dead battery.

The screen is using 53.8 - 20.8 = 33ma.
That's 33 / 53.8 = 62% of the battery's current draw when booting and 33 / 51 = 64% when idling!

So of the 87ma of USB current, the battery is getting:
With the screen: 87 - 53.8 = 33.2ma or 33.2 / 87 = 38% of the USB current.
Without the screen: 87 - 20.8 = 66.2ma or 66.2 / 87 = 76% of the USB current.

I verified the calculators pull the same amount of current from the USB port no matter whether the screen is present or not.

Therefor, I conclude these tests show the battery charge rate when booting can be doubled by removing the screen. Since, without the screen, over half the power is being put into the battery instead of booting the calculator this might allow some calculators to revive themselves. I haven't intentionally caused the boot loop problem on my calculators so I don't know for certain that removing the screen will solve the problem, but I have high hopes for this theory.
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